Auto merge of #29383 - petrochenkov:empstr, r=pnkfelix

Fixes rust-lang/rust#28692
Fixes rust-lang/rust#28992
Fixes some other similar issues (see the tests)

[breaking-change], needs crater run (cc @brson or @alexcrichton )

The pattern with parens `UnitVariant(..)` for unit variants seems to be popular in rustc (see the second commit), but mostly used by one person (@nikomatsakis), according to git blame. If it causes breakage on crates.io I'll add an exceptional case for it.

clean/mod.rs

@@ -774,7 +774,7 @@ impl Clean<Option<Lifetime>> for ty::Region {
  ty::ReScope(..) |
  ty::ReVar(..) |
  ty::ReSkolemized(..) |
- ty::ReEmpty(..) => None
+ ty::ReEmpty => None
  }
  }
 }
@@ -1607,7 +1607,7 @@ impl Clean<Type> for hir::Ty {
  TyPolyTraitRef(ref bounds) => {
  PolyTraitRef(bounds.clean(cx))
  },
- TyInfer(..) => {
+ TyInfer => {
  Infer
  },
  TyTypeof(..) => {